Since high-temperature superconductivity with onset at 30.degree. K. was discovered for a mixed structure in the La-Ba-Cu-O system (1), a great deal of effort has been devoted to the synthesis of a new oxide superconductor exhibiting a high critical temperature (T.sub.c). Thus, high critical temperatures of 92.degree. K. and 95.degree. K. were recently obtained in oxide compounds based on the Y-Ba-Cu-O (2) and Yb-Ba-Cu-O (3) systems, respectively. The crystal structure of these compounds has been identified as that of oxygen deficient perovskite, tetragonal K.sub.2 NiF.sub.4.
Subsequently, non-rare earth type superconducting oxides containing bismuth were discovered (4, 5, 6) and found to have superconducting dual phases which are responsible for a high T.sub.c of 105.degree. K. and a low T.sub.c of 75.degree. K., respectively. Initially, the crystal structure of these phases was reportedly indexed as an orthorhomic structure (pseudo tetragonal structure) in which lattice parameters a, b are very similar, i.e., a.about.b (7, 8, 9, 10). However, a high resolution electron microscopy study on the Bismuth oxides showed that the orthorhombic cell has a modulated structure along the a-axis which is five times the subcell size b.sub.sc, i.e., b.about.5b.sub.sc and b=27 .ANG.(11, 12, 13). All the observed lattice parameters are approximately within the range of a.about.5.41+0.015 .ANG., b.about.5.4+015 .ANG.; c.about.30.8+0.1 .ANG..
The high T.sub.c associated with these oxides represents a potential for significant technological applications. High T.sub.c oxide superconductors have been prepared through many processes of blending, pressing and sintering and the shape is usually in bulk form. For many significant technological applications, it is desired to fabricate superconducting oxides in wire or filament form. However, processing and fabrication of superconducting oxides into desired forms such as wire and filament are accompanied by serious problems due to inherent material properties such as brittleness, anisotropy in current flow, weak link behavior caused by defect structures, etc. In an effort to alleviate these problems, diversified approaches to processing and fabrication have been explored.
One such approach is the "precursor alloy-oxidation route" in which precursor alloys containing the constituent elements, with the exception of oxygen, of the superconducting oxide are prepared by rapid solidification (14, 15, 16) or mechanical alloying (17, 18) followed by oxidation at high temperatures for a prolonged period of time in order to form the superconducting orthorhmobic perovskite structure. While the precursor alloys are homogeneous and often show amorphous structures, the resulting superconducting oxides are still inherently brittle and therefore not suited for fabrication into wire or filament.
In another approach, superconducting oxide powders are compacted into a metal tube, sealed under vacuum and then extruded into wires at high temperatures. However, the as-extruded wires are not superconducting because the powders have lost oxygen during the high temperature extrusion process. Therefore, oxygen must be replenished by annealing at intermediate temperatures so that the powders can regain superconductivity by absorption of oxygen which penetrates through the metal wall of the wire to the powders. However, the time required for oxygen to penetrate the metal wall of the wire to reach the powders is too long to be practical. Alternatively, the metal wall of the wire can be chemically etched to expose the powders and the wire is then immersed in an oxygen atmosphere. However, this alternative is too costly to be economically practical.
In still another process, the addition of up to 50 weight percent of silver (19) or gold (20) to the precursor alloys in rare earth metal systems and the addition of up to 60 weight percent of silver in the bismuth system (21) have improved ductility in the resultant superconducting oxides. Notwithstanding this improvement in ductility, only ribbons of limited length. A greater improvement in ductility is required to obtain longer ribbons and wires. However, in the rare earth metal system, it has been found that a composite formed by oxidation of a Eu-Ba-Cu precursor alloy containing 70 weight percent gold was not a superconductor because the oxide particles were too widely separated. In other words, the discontinuity of the oxide phase prevented superconductivity.
Thus, a significant advance in the art would be achieved by providing a composite simultaneously possessing the ductility required for practical applications (e.g. wire or ribbon of substantial length) and a continuous superconducting oxide phase required for superconductivity.